Compositions and methods for storing holographic data

ABSTRACT

The present invention provides a method for storing holographic data comprising providing a holographic storage medium comprising an optically transparent substrate including a photochemically active dye having at least two nitrone groups; and irradiating the optically transparent substrate with a holographic interference pattern, wherein the pattern has a first wavelength and an intensity both sufficient to convert, within a volume element of the substrate, at least some of the photochemically active dye into a photo-product, and producing within the irradiated volume element concentration variations of the photo-product corresponding to the holographic interference pattern, thereby producing a first optically readable datum corresponding to the volume element.

RELATED APPLICATIONS

This non-provisional application is related to U.S. provisionalapplication having Ser. No. 60/974,868, filed Sep. 25, 2007.

BACKGROUND

The invention relates to methods for storing holographic data. Further,the present disclosure relates to holographic data storage media andarticles, which are derived from these methods. In addition the presentdisclosure relates to thiophene-containing polynitrones dyes.

Holographic storage is the storage of data in the form of holograms,which are images of three dimensional interference patterns created bythe intersection of two beams of light, in a photosensitive medium. Thesuperposition of a signal beam, which contains digitally encoded data,and a reference beam forms an interference pattern within the volume ofthe medium resulting in a chemical reaction that changes or modulatesthe refractive index of the medium. This modulation serves to record asthe hologram both the intensity and phase information from the signal.The hologram can later be retrieved by exposing the storage medium tothe reference beam alone, which interacts with the stored holographicdata to generate a reconstructed signal beam proportional to the initialsignal beam used to store the holographic image. Thus, in holographicdata storage, data is stored throughout the volume of the medium viathree dimensional interference patterns.

Each hologram may contain anywhere from one to 1×10⁶ or more bits ofdata. One distinct advantage of holographic storage over surface-basedstorage formats, including CDs or DVDs, is that a large number ofholograms may be stored in an overlapping manner in the same volume ofthe photosensitive medium using a multiplexing technique, such as byvarying the signal and/or reference beam angle, wavelength, or mediumposition. However, a major impediment towards the realization ofholographic storage as a viable technique has been the development of areliable and economically feasible storage medium.

Early holographic storage media employed inorganic photo-refractivecrystals, such as doped or un-doped lithium niobate (LiNbO₃), in whichincident light creates refractive index changes. These refractive indexchanges are due to the photo-induced creation and subsequent trapping ofelectrons leading to an induced internal electric field that ultimatelymodifies the refractive index through a linear electro-optic effect.However, LiNbO₃ is expensive, exhibits relatively poor efficiency, fadesover time, and requires thick crystals to observe any significant indexchanges.

Therefore, there is a need for improved holographic data storage methodsand materials through which enhanced holographic data storage capacitiescan be achieved. Further, there is also a need for methods to enhancethe lifetime of the stored holographic data, such that for example, thedata is not erased thermally, or when ambient light is incident on thedata storage medium, or during read-out.

BRIEF DESCRIPTION

In one aspect, the present invention provides a method for storingholographic data, said method comprising:

(A) providing a holographic storage medium comprising an opticallytransparent substrate, said optically transparent substrate comprising aphotochemically active dye having at least two nitrone groups; and

(B) irradiating the optically transparent substrate with a holographicinterference pattern, wherein the pattern has a first wavelength and anintensity both sufficient to convert, within a volume element of thesubstrate, at least some of the photochemically active dye into aphoto-product, and producing within the irradiated volume elementconcentration variations of the photo-product corresponding to theholographic interference pattern, thereby producing an opticallyreadable datum corresponding to the volume element.

In another aspect, the present invention provides a method for storingholographic data, said method comprising:

(A) providing a holographic storage medium comprising an opticallytransparent substrate, said optically transparent substrate comprising aphotochemically active dye having structure (I)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; Q¹ is a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical, or a polymer chain; and “a” is aninteger from 2 to 100; and

(B) irradiating the optically transparent substrate with a holographicinterference pattern, wherein the pattern has a first wavelength and anintensity both sufficient to convert, within a volume element of thesubstrate, at least some of the photochemically active dye into aphoto-product, and producing within the irradiated volume elementconcentration variations of the photo-product corresponding to theholographic interference pattern, thereby producing an opticallyreadable datum corresponding to the volume element.

In still yet another aspect, the present invention provides a method forstoring holographic data, said method comprising:

(A) providing a holographic storage medium comprising an opticallytransparent substrate, said optically transparent substrate comprising aphotochemically active dye having structure (II)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and “a” is an integer from 2 to 4, wherein the dye is presentin an amount from about 0.1 weight percent to about 10 weight percent;and

(B) irradiating the optically transparent substrate with a holographicinterference pattern, wherein the pattern has a first wavelength and anintensity both sufficient to convert, within a volume element of thesubstrate, at least some of the photochemically active dye into aphoto-product, and producing within the irradiated volume elementconcentration variations of the photo-product corresponding to theholographic interference pattern, thereby producing an opticallyreadable datum corresponding to the volume element, and wherein thefirst wavelength is about 500 nm.

In yet another embodiment, the present invention provides a novelthiophene-containing polynitrone compound having structure (II).

These and other features, aspects, and advantages of the presentinvention may be understood more readily by reference to the followingdetailed description.

DETAILED DESCRIPTION

In the following specification and the claims, which follow, referencewill be made to a number of terms, which shall be defined to have thefollowing meanings.

The singular forms “a”, “an” and “the” include plural referents unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

As used herein, the term “solvent” can refer to a single solvent or amixture of solvents.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, is not to be limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value.

As used herein, the term “aromatic radical” refers to an array of atomshaving a valence of at least one comprising at least one aromatic group.The array of atoms having a valence of at least one comprising at leastone aromatic group may include heteroatoms such as nitrogen, sulfur,selenium, silicon and oxygen, or may be composed exclusively of carbonand hydrogen. As used herein, the term “aromatic radical” includes butis not limited to phenyl, pyridyl, furanyl, thienyl, naphthyl,phenylene, and biphenyl radicals. As noted, the aromatic radicalcontains at least one aromatic group. The aromatic group is invariably acyclic structure having 4n+2 “delocalized” electrons where “n” is aninteger equal to 1 or greater, as illustrated by phenyl groups (n=1),thienyl groups (n=1), furanyl groups (n=1), naphthyl groups (n=2),azulenyl groups (n=2), anthraceneyl groups (n=3) and the like. Thearomatic radical may also include nonaromatic components. For example, abenzyl group is an aromatic radical, which comprises a phenyl ring (thearomatic group) and a methylene group (the nonaromatic component).Similarly a tetrahydronaphthyl radical is an aromatic radical comprisingan aromatic group (C₆H₃) fused to a nonaromatic component —(CH₂)₄—. Forconvenience, the term “aromatic radical” is defined herein to encompassa wide range of functional groups such as alkyl groups, alkenyl groups,alkynyl groups, haloalkyl groups, haloaromatic groups, conjugated dienylgroups, alcohol groups, ether groups, aldehyde groups, ketone groups,carboxylic acid groups, acyl groups (for example carboxylic acidderivatives such as esters and amides), amine groups, nitro groups, andthe like. For example, the 4-methylphenyl radical is a C₇ aromaticradical comprising a methyl group, the methyl group being a functionalgroup which is an alkyl group. Similarly, the 2-nitrophenyl group is aC₆ aromatic radical comprising a nitro group, the nitro group being afunctional group. Aromatic radicals include halogenated aromaticradicals such as 4-trifluoromethylphenyl,hexafluoroisopropylidenebis(4-phen-1-yloxy) (i.e., —OPhC(CF₃)₂PhO—),4-chloromethylphen-1-yl, 3-trifluorovinyl-2-thienyl,3-trichloromethylphen-1-yl (i.e., 3-CCl₃Ph—),4-(3-bromoprop-1-yl)phen-1-yl (i.e., 4-BrCH₂CH₂CH₂Ph—), and the like.Further examples of aromatic radicals include 4-allyloxyphen-1-oxy,4-aminophen-1-yl (i.e., 4-H₂NPh—), 3-aminocarbonylphen-1-yl (i.e.,NH₂COPh—), 4-benzoylphen-1-yl, dicyanomethylidenebis(4-phen-1-yloxy)(i.e., -OPhC(CN)₂PhO—), 3-methylphen-1-yl, methylenebis(4-phen-1-yloxy)(i.e., —OPhCH₂PhO—), 2-ethylphen-1-yl, phenylethenyl,3-formyl-2-thienyl, 2-hexyl-5-furanyl,hexamethylene-1,6-bis(4-phen-1-yloxy) (i.e., —OPh(CH₂)₆PhO—),4-hydroxymethylphen-1-yl (i.e., 4-HOCH₂Ph—), 4-mercaptomethylphen-1-yl(i.e., 4-HSCH₂Ph—), 4-methylthiophen-1-yl (i.e., 4-CH₃SPh—),3-methoxyphen-1-yl, 2-methoxycarbonylphen-1-yloxy (e.g., methylsalicyl), 2-nitromethylphen-1-yl (i.e., 2-NO₂CH₂Ph),3-trimethylsilylphen-1-yl, 4-t-butyldimethylsilylphenl-1-yl,4-vinylphen-1-yl, vinylidenebis(phenyl), and the like. The term “aC₃-C₁₀ aromatic radical” includes aromatic radicals containing at leastthree but no more than 10 carbon atoms. The aromatic radical1-imidazolyl (C₃H₂N₂—) represents a C₃ aromatic radical. The benzylradical (C₇H₇—) represents a C₇ aromatic radical.

As used herein the term “cycloaliphatic radical” refers to a radicalhaving a valence of at least one, and comprising an array of atoms whichis cyclic but which is not aromatic. As defined herein a “cycloaliphaticradical” does not contain an aromatic group. A “cycloaliphatic radical”may comprise one or more noncyclic components. For example, acyclohexylmethyl group (C₆H₁₁CH₂—) is a cycloaliphatic radical whichcomprises a cyclohexyl ring (the array of atoms which is cyclic butwhich is not aromatic) and a methylene group (the noncyclic component).The cycloaliphatic radical may include heteroatoms such as nitrogen,sulfur, selenium, silicon and oxygen, or may be composed exclusively ofcarbon and hydrogen. For convenience, the term “cycloaliphatic radical”is defined herein to encompass a wide range of functional groups such asalkyl groups, alkenyl groups, alkynyl groups, haloalkyl groups,conjugated dienyl groups, alcohol groups, ether groups, aldehyde groups,ketone groups, carboxylic acid groups, acyl groups (for examplecarboxylic acid derivatives such as esters and amides), amine groups,nitro groups, and the like. For example, the 4-methylcyclopent-1-ylradical is a C₆ cycloaliphatic radical comprising a methyl group, themethyl group being a functional group which is an alkyl group.Similarly, the 2-nitrocyclobut-1-yl radical is a C₄ cycloaliphaticradical comprising a nitro group, the nitro group being a functionalgroup. A cycloaliphatic radical may comprise one or more halogen atoms,which may be the same or different. Halogen atoms include, for example;fluorine, chlorine, bromine, and iodine. Cycloaliphatic radicalscomprising one or more halogen atoms include2-trifluoromethylcyclohex-1-yl, 4-bromodifluoromethylcyclooct-1-yl,2-chlorodifluoromethylcyclohex-1-yl, hexafluoroisopropylidene-2,2-bis(cyclohex-4-yl) (i.e., -C₆H₁₀C(CF₃)₂ C₆H₁₀—),2-chloromethylcyclohex-1-yl, 3-difluoromethylenecyclohex-1-yl,4-trichloromethylcyclohex-1-yloxy,4-bromodichloromethylcyclohex-1-ylthio, 2-bromoethylcyclopent-1-yl,2-bromopropylcyclohex-1-yloxy (e.g., CH₃CHBrCH₂C₆H₁₀O—), and the like.Further examples of cycloaliphatic radicals include4-allyloxycyclohex-1-yl, 4-aminocyclohex-1-yl (i.e., H₂NC₆H₁₀—),4-aminocarbonylcyclopent-1-yl (i.e., NH₂COC₅H₈—),4-acetyloxycyclohex-1-yl, 2,2-dicyanoisopropylidenebis(cyclohex-4-yloxy)(i.e., —OC₆H₁₀C(CN)₂C₆H₁₀—), 3-methylcyclohex-1-yl,methylenebis(cyclohex-4-yloxy) (i.e., —OC₆H₁₀CH₂C₆H₁₀O—),1-ethylcyclobut-1-yl, cyclopropylethenyl, 3-formyl-2-terahydrofuranyl,2-hexyl-5-tetrahydrofuranyl, hexamethylene-1,6-bis(cyclohex-4-yloxy)(i.e., —OC₆H₁₀(CH₂)₆C₆H₁₀O—), 4-hydroxymethylcyclohex-1-yl (i.e.,4-HOCH₂C₆H₁₀—), 4-mercaptomethylcyclohex-1-yl (i.e., 4-HSCH₂C₆H₁₀—),4-methylthiocyclohex-1-yl (i.e., 4-CH₃SC₆H₁₀—), 4-methoxycyclohex-1-yl,2-methoxycarbonylcyclohex-1-yloxy (2-CH₃OCOC₆H₁₀O—),4-nitromethylcyclohex-1-yl (i.e., NO₂CH₂C₆H₁₀—),3-trimethylsilylcyclohex-1-yl, 2-t-butyldimethylsilylcyclopent-1-yl,4-trimethoxysilylethylcyclohex-1-yl (e.g., (CH₃O)₃SiCH₂CH₂C₆H₁₀—),4-vinylcyclohexen-1-yl, vinylidenebis(cyclohexyl), and the like. Theterm “a C₃-C₁₀ cycloaliphatic radical” includes cycloaliphatic radicalscontaining at least three but no more than 10 carbon atoms. Thecycloaliphatic radical 2-tetrahydrofuranyl (C₄H₇O—) represents a C₄cycloaliphatic radical. The cyclohexylmethyl radical (C₆H₁₁CH₂—)represents a C₇ cycloaliphatic radical.

As used herein the term “aliphatic radical” refers to an organic radicalhaving a valence of at least one consisting of a linear or branchedarray of atoms which is not cyclic. Aliphatic radicals are defined tocomprise at least one carbon atom. The array of atoms comprising thealiphatic radical may include heteroatoms such as nitrogen, sulfur,silicon, selenium and oxygen or may be composed exclusively of carbonand hydrogen. For convenience, the term “aliphatic radical” is definedherein to encompass, as part of the “linear or branched array of atomswhich is not cyclic” a wide range of functional groups such as alkylgroups, alkenyl groups, alkynyl groups, haloalkyl groups, conjugateddienyl groups, alcohol groups, ether groups, aldehyde groups, ketonegroups, carboxylic acid groups, acyl groups (for example carboxylic acidderivatives such as esters and amides), amine groups, nitro groups, andthe like. For example, the 4-methylpent-1-yl radical is a C₆ aliphaticradical comprising a methyl group, the methyl group being a functionalgroup which is an alkyl group. Similarly, the 4-nitrobut-1-yl group is aC₄ aliphatic radical comprising a nitro group, the nitro group being afunctional group. An aliphatic radical may be a haloalkyl group whichcomprises one or more halogen atoms which may be the same or different.Halogen atoms include, for example; fluorine, chlorine, bromine, andiodine. Aliphatic radicals comprising one or more halogen atoms includethe alkyl halides trifluoromethyl, bromodifluoromethyl,chlorodifluoromethyl, hexafluoroisopropylidene, chloromethyl,difluorovinylidene, trichloromethyl, bromodichloromethyl, bromoethyl,2-bromotrimethylene (e.g., —CH₂CHBrCH₂—), and the like. Further examplesof aliphatic radicals include allyl, aminocarbonyl (i.e., —CONH₂),carbonyl, 2,2-dicyanoisopropylidene (i.e., —CH₂C(CN)₂CH₂—), methyl(i.e., —CH₃), methylene (i.e., —CH₂—), ethyl, ethylene, formyl (i.e.,—CHO), hexyl, hexamethylene, hydroxymethyl (i.e., —CH₂OH),mercaptomethyl (i.e., —CH₂SH), methylthio (i.e., —SCH₃),methylthiomethyl (i.e., —CH₂SCH₃), methoxy, methoxycarbonyl (i.e.,CH₃OCO—), nitromethyl (i.e., —CH₂NO₂), thiocarbonyl, trimethylsilyl (i.e., (CH₃)₃Si—), t-butyldimethylsilyl, 3-trimethyoxysilylpropyl (i.e.,(CH₃O)₃SiCH₂CH₂CH₂—), vinyl, vinylidene, and the like. By way of furtherexample, a C₁-C₁₀ aliphatic radical contains at least one but no morethan 10 carbon atoms. A methyl group (i.e., CH₃—) is an example of a C₁aliphatic radical. A decyl group (i.e., CH₃(CH₂)₉—) is an example of aC₁₀ aliphatic radical.

As defined herein, the term “optically transparent” as applied to anoptically transparent substrate or an optically transparent plasticmaterial means that the substrate or plastic material has an absorbanceof less than 1. That is, at least 10 percent of incident light istransmitted through the material at at least one wavelength in a rangebetween about 300 nanometers and about 1500 nanometers. For example,when configured as a film having a thickness suitable for use inholographic data storage said film exhibits an absorbance of less than 1at at least one wavelength in a range between about 300 nanometers andabout 1500 nanometers.

As used herein, the terms “photochemically reactive” and“photochemically active” have the same meaning and are interchangeableterms.

As defined herein, the term “volume element” means a three dimensionalportion of a total volume.

As defined herein, the term “optically readable datum” can be understoodas a datum that is stored as a hologram patterned within one or morevolume elements of an optically transparent substrate.

As noted, holographic data storage relies upon the introduction oflocalized variations in the refractive index of the opticallytransparent substrate comprising the photochemically active dye as ameans of storing holograms. The refractive index within an individualvolume element of the optically transparent substrate may be constantthroughout the volume element, as in the case of a volume element thathas not been exposed to electromagnetic radiation, or in the case of avolume element in which the photochemically active dye has been reactedto the same degree throughout the volume element. It is believed thatmost volume elements that have been exposed to electromagnetic radiationduring the holographic data writing process will contain a complexholographic pattern, and as such, the refractive index within the volumeelement will vary across the volume element. In instances in which therefractive index within the volume element varies across the volumeelement, it is convenient to regard the volume element as having an“average refractive index” which may be compared to the refractive indexof the corresponding volume element prior to irradiation. Thus, in oneembodiment an optically readable datum comprises at least one volumeelement having a refractive index that is different from a (the)corresponding volume element of the optically transparent substrateprior to irradiation. Data storage is achieved by locally changing therefractive index of the data storage medium in a graded fashion(continuous sinusoidal variations), rather than discrete steps, and thenusing the induced changes as diffractive optical elements.

In one embodiment of the invention, a holographic storage mediumcomprising an optically transparent substrate is provided. The opticallytransparent substrate may be made of materials possessing sufficientoptical quality such as, low scatter, low birefringence, and negligiblelosses at the wavelengths of interest, to render the data stored in theholographic storage medium readable. Generally, plastic materials thatexhibit these properties may be used as the substrate. However, theplastic materials should be capable of withstanding the particularprocessing parameters employed (e.g., inclusion of the dye, exposure toa sensitizing solvent and application of any coating or subsequentlayers, and molding it into a final format) and subsequent storageconditions. In one embodiment, the optically transparent plasticmaterials may comprise organic polymers such as, for example, oligomers,polymers, dendrimers, ionomers, copolymers such as block copolymers,random copolymers, graft copolymers, star block copolymers, and thelike, or a combination comprising at least one of the foregoingpolymers. In one embodiment, the optically transparent substratecomprises a polycarbonate.

The photochemically active dye is one which renders the opticallytransparent substrate capable of having holograms “written” into it at afirst wavelength. And further, the photochemically active dye should besuch that a hologram having been “written” into the opticallytransparent substrate at a first wavelength is not erased when thehologram is “read”. It is desirable to use dyes that enable “writing” ofthe holographic interference pattern into the optically transparentsubstrate at a wavelength in a range from about 300 nm to about 800 nm.

In one embodiment, the photochemically active dye has an opticalabsorption resonance characterized by a center wavelength associatedwith the maximum absorption and a spectral width (full width at half ofthe maximum, FWHM) of less than 500 nanometers. Typically, thephotochemically active dyes undergo a light induced chemical reactionwhen exposed to light with a wavelength within the absorption range toform at least one photo-product. This reaction can be aphoto-decomposition reaction, such as oxidation, reduction, or bondbreaking to form smaller constituents, or a molecular rearrangement,such as a sigmatropic rearrangement, or addition reactions includingpericyclic cycloadditions. Thus in an embodiment, data storage in theform of holograms is achieved wherein the photo-product is patternedwithin the optically transparent substrate to provide the at least oneoptically readable datum.

In an embodiment, the photochemically active dye is a compoundcomprising at least two nitrone groups having a structure (I)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; Q¹ is a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical, or a polymer chain; and “a” is aninteger from 2 to 100.

Representative polynitrone compounds encompassed by generic structure(I) are illustrated in Table I. One of ordinary skill in the art willappreciate the relationship between generic structure (I) and theindividual structures of Entries 1a-1f of Table I.

TABLE I Compounds Exemplifying The Photochemically Active Dye Having . .. Structure (I) Example Structure Comment 1a

R² = hydrogen;  a = 2. 1b

R² = hydrogen;  a = 2. 1c

R² = hydrogen;  a = 2. 1d

R² = hydrogen;  a = 2. 1e

R² = hydrogen;  a = 2. 1f

R² = hydrogen;  a = 2.

In one embodiment, the present invention photochemically active dye is athe novel thiophene-containing polynitrone having structure (II)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and “a” is an integer from 2 to 4.

Representative thiophene-containing polynitrones encompassed by genericstructure (II) are illustrated in Table II. One of ordinary skill in theart will appreciate the relationship between generic structure (II) andthe individual structures of Entries 2a-2e of Table II.

TABLE II Compounds Exemplifying Thiophene-containing Polynitrones HavingStructure (II) Entry Structure 2a

 R¹ =

 R² = H“a” = 2 2b

 R¹ = phenyl R² = H“a” = 2 2c

 R¹ =

 R² = H“a” = 2 2d

 R¹ = phenyl R² = H“a” = 3 2e

 R¹ = C₃H₇ aliphatic group and R² = H

In one embodiment, the nitrone moieties (III)

wherein R¹ is a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical; and R² is a hydrogen, adeuterium, a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical,or a C₂-C₃₀ aromatic radical, are attached to positions 2 and 3, of thethiophene moiety (IV)

In another embodiment, the nitrone moieties (III) are attached topositions 2 and 4, of the thiophene moiety (IV). In yet anotherembodiment, the nitrone moieties (III) are attached to positions 2 and5, of the thiophene moiety (IV). In yet another embodiment, the nitronemoieties (III) are attached to positions 3 and 4, of the thiophenemoiety (IV).

In another embodiment of the present invention, the photochemicallyactive dye is a novel thiophene-containing dinitrone having structure(V)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and R³ and R⁴ are independently at each occurrence halogen,hydrogen, deuterium, a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical.

In one embodiment, the nitrone moieties (III) are attached to positions2 and 3, of the thiophene moiety (VI)

wherein R³ and R⁴ are independently at each occurrence halogen, ahydrogen, a deuterium, a C₁-C₂₀ aliphatic radical, a C₃-C₂₀cycloaliphatic radical, or a C₂-C₃₀ aromatic radical. In one embodiment,the nitrone moieties (III) are attached to positions 2 and 4, of thethiophene moiety (VI). In another embodiment, the nitrone moieties (III)are attached to positions 2 and 5, of the thiophene moiety (VI). In yetanother embodiment, the nitrone moieties (III) are attached to positions3 and 4, of the thiophene moiety (VI).

In one embodiment, the present invention provides a thiophene-containingdinitrone having structure (VII)

wherein R³ and R⁴ are independently halogen, hydrogen, deuterium, aC₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀aromatic radical; and R⁵ is independently at each occurrence a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical.

In another embodiment, the present invention provides athiophene-containing dinitrone having structure (VIII)

wherein R³ and R⁴ are independently halogen, hydrogen, deuterium, aC₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀aromatic radical; and R⁵ is independently at each occurrence a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical.

In another embodiment, the present invention provides athiophene-containing dinitrone having structure (IX).

The novel thiophene-containing polynitrones and thiophene-containingdinitrones provided by the present invention may be prepared by avariety of methods. In one embodiment, thiophene polynitrones areprepared by contacting a carbonyl compound and a hydroxylamine compound.In one embodiment, the hydroxylamine is a polymer comprisinghydroxylamine groups. In one embodiment, the reaction (“contacting”)between the carbonyl compound and the hydroxylamine compound can becarried out at room temperature. In an alternate embodiment, thereaction between the carbonyl compound and the hydroxylamine compoundcan be carried out at a temperature in a range from about 0° C. to about50° C. In another embodiment, the reaction can be carried out in asolvent at a temperature in excess of 100° C. with the removal of waterformed as a by-product in the condensation reaction. In anotherembodiment, the reaction is carried out in an organic solvent attemperature in a range from about 120° C. to about 160° C. In yetanother embodiment, the reaction is carried out in a melt. In certaininstances it may be advantageous to conduct the reaction in the presenceof a catalyst Suitable solvents include oDCB (orthodichorobenzene),toluene, xylene, chlorobenzene, methylene chloride, anisole, veratrole,alkyl alcohols such as ethanol and methanol, alkanoic acids such asacetic acid, and combinations thereof. The experimental section of thisdisclosure provides a number of specific methods and conditions for thepreparation of novel thiophene-containing polynitrones provided by thepresent invention.

In one embodiment, the photochemically active dye (sometimes referred toas “the dye”) utilized in the present invention has a narrow absorptionband, which undergo a chemical change upon exposure to certain “write”wavelengths of light. The photochemically active narrow band dye isdefined as having an absorption spectrum which is characterized by acenter wavelength associated with the maximum absorption and a spectralwidth (full width at half of the maximum, FWHM) of less than about 500nanometers. The photo-product or photo-products which result frominteraction of the photochemically active dye with light having the“write” wavelength typically exhibits an absorption spectrum which isentirely different from that exhibited by the dye prior to irradiation.The chemical change in the dye produced by interaction with light of thewrite wavelength produces a corresponding change in the molecularstructure of the dye, thereby producing a “photo-product”. Thismodification to the structure of the dye molecule and concurrent changesin the light absorption properties of the photo-product(s) relative tothe starting dye produces a significant change in refractive indexwithin the substrate that can be observed at a “read” wavelength.

In one embodiment, upon exposure to the holographic interference patternhaving a first wavelength and intensity sufficient to record at leastone optically readable datum, the photoproduct of the photochemicallyactive dye dispersed in an optically transparent substrate comprises anoxaziridine, a rearrangement product of an oxaziridine, or a combinationthereof.

In one embodiment, the photoproduct comprises an oxaziridine havingstructure (X)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;and R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical.

In another embodiment, the photoproduct comprises an oxaziridine havingstructure (XI)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and R³ and R⁴ are independently halogen, hydrogen, deuterium, aC₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀aromatic radical.

In one embodiment, the photoproduct comprises a mono oxaziridinecompound. In some embodiments the photocyclization of thephotochemically active polynitrone dye to an oxaziridine photoproductproceeds with a high quantum efficiency, and a large refractive indexchange. Typically, the photocyclization is induced in only a portion ofthe total amount of the photochemically active polynitrone dye presentin a given volume element, thus providing a refractive index contrastbetween the unconverted dye and the oxaziridine photo-product, andproviding the concentration variations of the photo-productcorresponding to the holographic interference pattern, and constitutingthe optically readable datum.

In one embodiment, as noted above, step (B) in the method for storingholographic data, comprises irradiating the optically transparentsubstrate with a holographic interference pattern, wherein the patternhas a first wavelength and an intensity both sufficient to convert,within a volume element of the substrate, at least some of thephotochemically active dye into a photo-product, and producing withinthe irradiated volume element concentration variations of thephoto-product corresponding to the holographic interference pattern,thereby producing an optically readable datum corresponding to thevolume element. The optically readable datum is stored in the opticallytransparent substrate as a hologram patterned within at least one volumeelement of the optically transparent substrate.

Those skilled in the art will appreciate that the lingeringphotosensitivity of the unconverted (residual) photochemically reactivedye presents a general problem that can adversely affect the integrityof the stored data if no step is taken to stabilize the unconvertedphotochemically reactive dye. In the case where the unconvertedphotochemically reactive dye is a polynitrone, protonation of thepolynitrone remaining following the recording of the holographic dataprovides an efficient means of preventing further conversion of thepolynitrone to photo-products under the influence of, for example, aread beam or ambient light.

As noted above, a photochemically active dye is disposed on theoptically transparent substrate. The photochemically active dye is one,which renders the optically transparent substrate capable of havingholograms “written” into it at a first wavelength. And further, thephotochemically active dye should be such that a hologram having been“written” into the optically transparent substrate at a first wavelengthis not erased when the hologram is “read”.

In an embodiment, the optically transparent substrate is irradiated witha holographic interference pattern having a first wavelength to recorddata. The optically transparent substrate is then irradiated withradiation having a second wavelength to stabilize the written data, andthe stabilized data can then be read using radiation having a thirdwavelength (e.g., a “read beam”), wherein the radiation at each step canindependently have a wavelength from about 300 nm to about 1,500 nm. Inan embodiment, the first, second, and third wavelengths can beindependently between about 300 nm and about 800 nm. In one embodiment,the first wavelength (or the writing wavelength) for writing andrecording the data onto the holographic data storage medium is fromabout 375 nm to about 450 nm. In another embodiment, the firstwavelength can be from about 450 nm to about 550 nm. In one embodiment,the first wavelength is in a range from about 375 nm to about 450 nm andthe second wavelength is in a range from about 450 to about 1500 nm. Inanother embodiment, the first wavelength is in a range from about 450 nmto about 550 nm and the second wavelength is in a range from about 550to about 1500 nm. In still another embodiment, the writing wavelength issuch that it is shifted by 0 nm to about 400 nm from the wavelength atwhich the recorded data is stabilized by the action of light of thesecond wavelength. Exemplary wavelengths at which writing and datastabilization are accomplished are about 405 nanometers (writing) andabout 532 nanometers (stabilization). The first wavelength is alsosometimes referred to as the “write” wavelength.

In one embodiment, the photochemically active dye is disposed within thesubstrate in an amount from about 0.1 weight percent to about 20 weightpercent. In some embodiments, the photochemically active dye is presentin an amount from about 5 weight percent to about 10 weight percent inthe substrate. In yet another embodiment, the photochemically active dyeis present in the substrate in an amount from about 15 weight percent toabout 20 weight percent. As used herein, the term “weight percent” ofthe dye refers to a ratio of the weight of the dye included in thesubstrate to the total weight of the substrate (inclusive of the weightof the dye). For example, 10 weight percent of the dye disposed in asubstrate implies 10 grams of the dye in 90 grams of the substrate. Theloading percentage of the dye may be controlled to provide desirableproperties.

Optically transparent plastic materials may be advantageously employedin the preparation of the optically transparent substrate. Opticallytransparent plastic materials used in producing holographic data storagemedia (such as the optically transparent substrate) can comprise anyplastic material having sufficient optical quality, e.g., low scatter,low birefringence, and negligible losses at the wavelengths of interest,to render the data in the holographic storage material readable. Organicpolymeric materials, such as for example, oligomers, polymers,dendrimers, ionomers, copolymers such as for example, block copolymers,random copolymers, graft copolymers, star block copolymers; and thelike, or a combination comprising at least one of the foregoing polymerscan be used. Thermoplastic polymers or thermosetting polymers can beused. Examples of suitable thermoplastic polymers include polyacrylates,polymethacrylates, polyamides, polyesters, polyolefins, polycarbonates,polystyrenes, polyesters, polyamideimides, polyaromaticates,polyaromaticsulfones, polyethersulfones, polyphenylene sulfides,polysulfones, polyimides, polyetherimides, polyetherketones, polyetheretherketones, polyether ketone ketones, polysiloxanes, polyurethanes,polyaromaticene ethers, polyethers, polyether amides, polyether esters,or the like, or a combination comprising at least one of the foregoingthermoplastic polymers. Some more possible examples of suitablethermoplastic polymers include, but are not limited to, amorphous andsemi-crystalline thermoplastic polymers and polymer blends, such as:polyvinyl chloride, linear and cyclic polyolefins, chlorinatedpolyethylene, polypropylene, and the like; hydrogenated polysulfones,ABS resins, hydrogenated polystyrenes, syndiotactic and atacticpolystyrenes, polycyclohexyl ethylene, styrene-acrylonitrile copolymer,styrene-maleic anhydride copolymer, and the like; polybutadiene,polymethylmethacrylate (PMMA), methyl methacrylate-polyimide copolymers;polyacrylonitrile, polyacetals, polyphenylene ethers, including, but notlimited to, those derived from 2,6-dimethylphenol and copolymers with2,3,6-trimethylphenol, and the like; ethylene-vinyl acetate copolymers,polyvinyl acetate, ethylene-tetrafluoroethylene copolymer, aromaticpolyesters, polyvinyl fluoride, polyvinylidene fluoride, andpolyvinylidene chloride. In one embodiment the optically transparentsubstrate comprises polycarbonate, for example bisphenol Apolycarbonate.

The optically transparent substrate may have a thickness tailored tomeet the demands of a particular intended usage of the storage medium.In one embodiment, the thickness of the storage medium is greater thanabout 100 micrometers. In some embodiments, the thickness may vary fromabout 100 micrometers to about 5 centimeters. For example, for use as aDVD or CD storage device typical thickness is about 600 micrometers toabout 1.2 millimeters. The shape of the optically transparent substrateincludes a variety of shapes such as, but not limited to, a square, arectangle, an oval or a circular shape.

The optically transparent substrate may comprise additional componentssuch as heat stabilizers; antioxidants; light stabilizers; plasticizers;antistatic agents; mold releasing agents; additional resins; binders,blowing agents; and the like, as well as combinations of the foregoingadditives.

Generally, the photochemically active polynitrone dyes and polymers usedfor forming the optically transparent substrate, and the holographicdata storage medium should be capable of withstanding the processingconditions used to prepare the holographic data storage medium, forexample during a step in which the photochemically active polynitroneand any additional additives which may be present are compounded with apolymer powder and subsequently molded into data storage discs. Invarious embodiments, the polynitrone dyes provided by the presentinvention exhibit enhanced thermal stability relative to knownphotochemically active dyes themselves useful in holographic datastorage applications.

In one embodiment, the present invention provides a holographic storagemedium comprising an optically transparent substrate comprising aphotochemically active polynitrone dye. In an embodiment, a film of anoptically transparent substrate comprising an optically transparentplastic material and at least one photochemically active polynitrone dyeis formed. Generally, the film is prepared by molding techniques using amolding composition that is obtained by mixing the polynitrone dye withan optically transparent plastic material. Mixing can be conducted inmachines such as a single or multiple screw extruder, a Buss kneader, aHenschel, a helicone, an Eirich mixer, a Ross mixer, a Banbury,kneaders, blenders and the like, a roll mill, molding machines such asinjection molding machines, vacuum forming machines, blow moldingmachine, or then like, or a combination comprising at least one of theforegoing machines. Alternatively, the polynitrone dye and the opticallytransparent plastic material may be dissolved in a solution and films ofthe optically transparent substrate can be formed from the solution.

In some embodiments, the photochemically active dye is dissolved in asolvent together with the polymer host to produce a solution. Films canbe made by spin-coating from this solution. In other embodiments, filmscan be formed by blade coating, substrate dipping, and spraying thedye/polymer solution. Suitable polymeric substrate materials containingthe photochemically active dye are at times referred to as “dopedpolymers”. Such doped polymers can be prepared by a variety oftechniques such as the solvent casting technique referred to above. Inone embodiment, the doped polymers can also be formed by dissolving thephotochemically active dye in a liquid monomer and thereafter thermallyor photoreactively polymerizing the monomer in the presence of thephotochemically active dye to produce an optically transparent substratematerial having dispersed uniformly within it the photochemically activedye. In another embodiment, such doped polymers is prepared by moldingor extrusion techniques of polymer/dye blends.

In one embodiment a data storage composition comprising aphotochemically active dinitrone, a thermoplastic polymer is injectionmolded to form an article that can be used for producing holographicdata storage media. The injection-molded article can have any geometry.Examples of suitable geometries include circular discs, square shapedplates, polygonal shapes, or the like. The thickness of the articles canvary, from being at least 100 micrometers in an embodiment, and at least250 micrometers in another embodiment. A thickness of at least 250micrometers is useful in producing holographic data storage disks thatare comparable to the thickness of current digital storage discs.

The molded data storage medium thus produced can be used for producingdata storage articles, which can be used for storing data in the form ofholograms. In one embodiment, the molded data storage medium isirradiated with a holographic interference pattern having a firstwavelength to record at least one optically readable datum and generateat least one photo-product of the photochemically active dye. Theoptically readable datum is stored as a hologram patterned within atleast one volume element of the data storage medium. Stabilization ofthe stored data may be achieved by various methods including the postdata recording application of a UV screener to the surface of the moldeddata storage medium. In an embodiment, the stabilized holographic datacan be read using radiation having a third wavelength. In an embodiment,this “read wavelength” can be between 350 and 1,100 nm.

The methods disclosed herein can be used for producing holographic datastorage media that can be used for bit-wise type data storage in anembodiment, and page-wise type storage of data in another embodiment. Instill another embodiment, the methods can be used for storing data inmultiple layers of the data storage medium. In view of the variousphotochemical transformations occurring with the polynitrones during thedata recording processes disclosed herein, it becomes possible toidentify a holographic data storage medium, or a holographic datastorage article comprising such a data storage medium, in terms of thechemical entities present before and after the data storage process.Thus in an embodiment, the present invention provides a holographic datastorage medium that can be used for storing data in the form ofholograms. Such a data storage medium comprises (i) at least oneoptically transparent plastic material, and (ii) at least onephotochemically active dye.

In another embodiment, the present invention provides a data storagemedium comprising (i) at least one optically transparent plasticmaterial, and (ii) at least one photochemically active dye havingstructure (I). In yet another embodiment, the present invention providesa data storage medium having at least one optically readable datumstored therein, the data storage medium comprising (i) at least oneoptically transparent plastic material, (ii) at least onephotochemically active dye having structure (I), and (iii) at least onephoto-stable product derived from the at least one photochemicallyactive dye, or combinations thereof, wherein the at least one opticallyreadable datum is stored as a hologram in the data storage medium.

In another embodiment, the present invention provides a data storagemedium comprising (i) at least one optically transparent plasticmaterial, and (ii) at least one photochemically active dye havingstructure (II). In yet another embodiment another embodiment, thepresent invention provides a data storage medium having at least oneoptically readable datum stored therein, the data storage mediumcomprising (i) at least one optically transparent plastic material, (ii)at least one photochemically active dye having structure (II), and (iii)at least one photo-stable product derived from the at least onephotochemically active dye, the at least one photo-product, orcombinations thereof, wherein the at least one optically readable datumis stored as a hologram in the data storage medium.

In various embodiments, the read wavelength is different from the writewavelength, such that at the wavelength selected for reading theinformation contained in the holographic storage medium there is verylittle or no absorption of the reading light. In one embodiment, thewavelength of light employed for reading is selected such that thedifference between the reading wavelength and the absorption bandassociated with the writing event is maximized. In one embodiment theread beam has a wavelength shifted from about 50 nm to about 400 nm fromthe write beam's wavelength. In some embodiments, a suitable read beamhas a wavelength from about 400 nm to about 800 nm. However, the fartheraway from the absorption band, the smaller the refractive index change,which negatively impacts the efficiency of the storage process. Inaddition, the greater the separation between the writing and readingwavelengths, the more difficult it may be to reconstruct the data. Thus,in some embodiments, reading wavelengths are usually selected as thenearest wavelength where the transmission is greater than 95%.

In some embodiments, blue light at wavelengths ranging from about 375 nmto about 425 nm may be used for writing and green/red light atwavelengths ranging from about 500 nm to about 800 nm may be used forreading. In other embodiments, the wavelength of light used for writingcan range from about 425 nm to about 550 nm, and the reading wavelengthcan range from about 600 nm to about 700 nm. In one embodiment, awavelength of 532 nm light can be used for writing and wavelengths ofeither 633 nm or 650 nm light can be used for reading.

Additional physical/optical concepts, which will aid one of ordinaryskill in the art to better appreciate the methods employed in thedevelopment of present invention are included herein. Thus, theabsorption cross section is a measurement of an atom or molecule'sability to absorb light at a specified wavelength, and is measured insquare cm/molecule. It is generally denoted by a (X) and is governed bythe Beer-Lambert Law for optically thin samples as shown in equation(1),

$\begin{matrix}{{\sigma (\lambda)} = {{{\ln (10)} \cdot \frac{{Absorbance}(\lambda)}{N_{o} \cdot L}}\left( {cm}^{2} \right)}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

wherein N₀ is the concentration in molecules per cubic centimeter, and Lis the sample thickness in centimeters.

Quantum efficiency (QE) is a measure of the probability of aphotochemical transition for each absorbed photon of a given wavelength.Thus, it gives a measure of the efficiency with which incident light isused to achieve a given photochemical conversion, also called as ableaching process. QE is given by equation (2),

$\begin{matrix}{{QE} = \frac{{hc}/\lambda}{\sigma \cdot F_{0}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

wherein “h” is the Planck's constant, “c” is the velocity of light, σ(λ)is the absorption cross section at the wavelength λ, and F₀ is thebleaching fluence. The parameter F₀ is given by the product of lightintensity (I) and a time constant (τ) that characterizes the bleachingprocess.

As noted, holographic data storage relies upon the introduction oflocalized variations in the refractive index of the opticallytransparent substrate comprising the photochemically active dye as ameans of storing holograms. The refractive index within an individualvolume element of the optically transparent substrate may be constantthroughout the volume element, as in the case of a volume element thathas not been exposed to electromagnetic radiation, or in the case of avolume element in which the photochemically active dye has been reactedto the same degree throughout the volume element. It is believed thatmost volume elements that have been exposed to electromagnetic radiationduring the holographic data writing process will contain a complexholographic pattern, and as such, the refractive index within the volumeelement will vary across the volume element. In instances in which therefractive index within the volume element varies across the volumeelement, it is convenient to regard the volume element as having an“average refractive index” which may be compared to the refractive indexof the corresponding volume element prior to irradiation. Thus, in oneembodiment an optically readable datum comprises at least one volumeelement having a refractive index that is different from a (the)corresponding volume element of the optically transparent substrateprior to irradiation. Data storage is achieved by locally changing therefractive index of the data storage medium in a graded fashion(continuous sinusoidal variations), rather than discrete steps, and thenusing the induced changes as diffractive optical elements.

As defined herein, the term M/# denotes the capacity of a data storagemedium, and can be measured as a function of the total number ofmultiplexed holograms that can be recorded at a volume element of thedata storage medium at a given diffraction efficiency. M/# depends uponvarious parameters, such as the change in refractive index (Δn), thethickness of the medium, and the dye concentration. These terms aredescribed further in this disclosure. The M/# is defined as shown inequation (3):

$\begin{matrix}{{M/\#} = {\sum\limits_{i = 1}^{N}\sqrt{\eta \; i}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

where η_(i) is the diffraction efficiency of the ith hologram, and N isthe number of recorded holograms. The experimental setup for M/#measurement for a test sample at a chosen wavelength, for example, at532 nanometers or 405 nanometers involves positioning the testing sampleon a rotary stage that is controlled by a computer. The rotary stage hasa high angular resolution, for example, about 0.0001 degree. A M/#measurement involves two steps: recording and readout. At recording,multiple planewave holograms are recorded at the same location on thesame sample. A plane wave hologram is a recorded interference patternproduced by a signal beam and a reference beam. The signal and referencebeams are coherent to each other. They are both planewaves that have thesame power and beam size, incident at the same location on the sample,and polarized in the same direction. Multiple planewave holograms arerecorded by rotating the sample. Angular spacing between two adjacentholograms is about 0.2 degree. This spacing is chosen so that theirimpact to the previously recorded holograms, when multiplexingadditional holograms, is minimal and at the same time, the usage of thetotal capacity of the media is efficient. Recording time for eachhologram is generally the same in M/# measurements. At readout, thesignal beam is blocked. The diffracted signal is measured using thereference beam and an amplified photo-detector. Diffracted power ismeasured by rotating the sample across the recording angle range with astep size of about 0.004 degree. The power of the reference beam usedfor readout is typically about 2-3 orders of magnitude smaller than thatused at recording. This is to minimize hologram erasure during readoutwhile maintaining a measurable diffracted signal. From the diffractedsignal, the multiplexed holograms can be identified from the diffractionpeaks at the hologram recording angles. The diffraction efficiency ofthe ith hologram, η_(i), is then calculated by using equation (4):

$\begin{matrix}{\eta_{i} = \frac{P_{i,{diffracted}}}{P_{reference}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

where P_(i, diffracted) is the diffracted power of the ith hologram. M/#is then calculated using the diffraction efficiencies of the hologramsand equation (3). Thus, a holographic plane wave characterization systemmay be used to test the characteristics of the data storage material,especially multiplexed holograms. Further, the characteristics of thedata storage material can also be determined by measuring thediffraction efficiency.

The capacity to store data as holograms (M/#) is also directlyproportional to the ratio of the change in refractive index per unit dyedensity (Δn/N0) at the wavelength used for reading the data to theabsorption cross section (σ) at a given wavelength used for writing thedata as a hologram. The refractive index change per unit dye density isgiven by the ratio of the difference in refractive index of the volumeelement before irradiation minus the refractive index of the same volumeelement after irradiation to the density of the dye molecules. Therefractive index change per unit dye density has a unit of(centimeter)³. Thus in an embodiment, the optically readable datumcomprises at least one volume element wherein the ratio of the change inthe refractive index per unit dye density of the at least one volumeelement to an absorption cross section of the at least onephotochemically active dye is at least about 10-5 expressed in units ofcentimeter.

Sensitivity (S) is a measure of the diffraction efficiency of a hologramrecorded using a certain amount of light fluence (F). The light fluence(F) is given by the product of light intensity (I) and recording time(τ). Mathematically, sensitivity is given by equation (5),

$\begin{matrix}{S = {\frac{\sqrt{\eta}}{I \cdot t \cdot L}\left( {{cm}\text{/}J} \right)}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

wherein I is the intensity of the recording beam, “t” is the recordingtime, L is the thickness of the recording (or data storage) medium(example, disc), and η is the diffraction efficiency. Diffractionefficiency is given by equation (6),

$\begin{matrix}{\eta = {\sin^{2}\frac{{\pi \cdot \Delta}\; {n \cdot L}}{\lambda \cdot {\cos (\theta)}}}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

wherein λ is the wavelength of light in the recording medium, θ is therecording angle in the media, and Δn is the refractive index contrast ofthe grating, which is produced by the recording process, wherein the dyemolecule undergoes a photochemical conversion.

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present invention to itsfullest extent. The following examples are included to provideadditional guidance to those skilled in the art in practicing theclaimed invention. The examples provided are merely representative ofthe work that contributes to the teaching of the present application.The following examples are intended only to illustrate methods andembodiments in accordance with the invention, and as such should not beconstrued as imposing limitations upon the claims.

EXAMPLES

Proton NMR spectra was measured using a 300 megahertz Bruker NMRspectrometer and d₆-dimethylsulfoxide or CDCl₃ as solvent. Compoundswere further characterized by a liquid chromatograph-mass spectrometer(LC-MS) system, comprising a liquid chromatograph and a Quattro UltimaPt mass spectrometer. An Xterra C18 (50 mm×4.6 mm; 5 microns) column wasused for the separating the components by liquid chromatography. Theseparated components were then analyzed by mass spectrometry.Ultraviolet-visible (UV-VIS) spectra were recorded using a double beamPerkin-Elmer Lambda 900 UV-VIS-NIR spectrophotometer. DifferentialScanning Chromatography (DSC) experiments were performed to study thethermal behavior of nitrones especially for melting or decompositiontemperature. The melting or decomposition temperatures were measured inpresence of nitrogen with a heating rate of 10° C./minute using DSCQ10(TA) instrument

Preparation Of 4-Carbethoxyphenylhydroxylamine 1

A solution of ethyl p-nitro benzoate (14.6 gm, 75 mmol) in 95% ethanol(75 ml) was mixed with ammonium chloride (4.6 gm, 86 mmol) in water (70ml) in 250 ml 3-necked flask. The resulting milky suspension was cooledon an ice bath and treated with zinc dust (10.9 gm, 167 mmol) inportions with stirring to keep the reaction mixture below 35° C. Afterabout two and half hours the zinc oxide was removed by filtration andrinsed with hot water followed by methylene chloride. The aqueousfiltrate was once again extracted with methylene chloride, and thecombined methylene chloride extracts were washed with brine, dried overanhydrous sulphate and evaporated under reduced pressure. An orange oilwas thus obtained (11.6 gm, 64 mmol, 85%). The crystallization of theoily product gave 5.6 gm (˜42% yield) purified4-carbethoxyphenylhydroxylamine (HPLC purity of 95.9% and a meltingpoint of 72.5° C.).

Example 1 Preparation Of 2,5-Bis (N-(4-Ethoxycarbonylphenyl)Imino-N-Oxide) Thiophene Ex. 1

Carbethoxyphenylhydroxylamine 1 (3.36 gm, 18.5 mmol) was mixed with 2,5thiophenedicarboxaldehyde (0.65 gm, 4.64 mmol) in glacial acetic acid(70 ml) and stirred at room temperature for 20 hours. The reactionmixture was poured into water (200 ml) and the product was filtered. Thefiltered product was washed with water and dried at 60° C. to give 2.0gm orange colored crude product. The product was purified by boiling thecrude product in acetonitrile followed by filtration gave 1.0 gm (˜46.0%yield) 2,5-bis (N-(4-ethoxycarbonylphenyl) imino-N-oxide) thiophene(HPLC purity: 94.3%; DSC: 247° C.).

Preparation Of 4-Nitro-Benzoic Acid 2-Ethyl-Hexyl Ester 2

To the 250 ml 3-necked flask was added 10.0 gm 2-ethyl 1-hexanol, 100 mldichloromethane, and 6.0 gm pyridine and the contents were stirred for10 minutes. To this mixture, 14.2 gm 4-nitro benzoyl chloride was addedslowly and stirred under reflux conditions for 2 hrs. The reactionmixture was then cooled, washed with 20% aqueous ammonium hydroxide. Theorganic layer was washed with 1N hydrochloric acid and brine, and driedover anhydrous sodium sulfate to give a crude product (˜65% yield).

Preparation of 4-Hydroxyamino-Benzoic Acid 2-Ethyl-Hexyl Ester 3

To a 500 ml 3-necked flask was added 14 gm p-nitro phenyl 2 ethyl hexylester, 50 ml ethanol, 3.1 gm ammonium chloride, and 50 ml water. To thismixture 7.3 gm zinc was added slowly and stirred at room temperature for5 hrs. The mixture was washed with methylenedichloride (MDC), thatseparated the organic layer. After distillation of MDC, solid wasobtained in a yield of 5.6 gm (42%).

Example 2 Preparation Of 2, 5-Thiophene Bis-2-EthylhexylesterphenylDinitrone (Ex.2)

To a 250 ml 3-necked flask was added 0.5 gm2,5-thiophenedicarboxaldehyde, 30 ml acetic acid, 5.6 gm2-ethylhexylesterphenyl hydroxylamine 3 and stirred for 20 hrs at roomtemperature. To the reaction mixture, 100 ml water was added and themixture was filtered. The filtered product was washed with water anddried. A yield of about 1.6 gm of 2, 5-thiophenebis-2-ethylhexylesterphenyl dinitrone of Ex.2 was obtained with a HPLCpurity of 98% (DSC: 205° C.).

Structures of the various nitrone and dinitrone dyes studied aregathered in Table 1 together with data for diffraction efficiency andquantum efficiency measured for molded disks containing the dye. Inaddition data on nitrone stability is also presented.

TABLE 1 Diffraction Quantum Efficiency Efficiency Decomposition at 532nm At 532 Temprature Dye Structure write/read nm (° C.) CEx. 1

— 0.0  110 CEx. 2

0.09 0.05 185 CEx. 3

— — 80   Ex. 3

0.42 0.18 247   Ex. 4

— — 251   Ex. 5

— — 210

Molded disks containing the nitrone or dinitrone dye were prepared asfollows. Molded disk for testing were prepared using a blend ofpolystyrene (PS1301) pellets that were ground to a coarse powder in aRetsch mill and dried in a circulating air oven at 80° C. for at least 4hours. In a Henschel mixer, 150 g of the dry polystyrene and 0.9 g ofthe dye (as given in Table 1) were blended to form a homogeneous powder.The disk was molded in a small injection molding machine (Mini-jector) a10 ton vertical injection machine, with a polished mold surface andmirrored stamper at a barrel temperature of about 200-210° C. to producedisks of 1.2 mm thickness and a diameter of 5.8 cm.

Diffraction efficiency was measured for the disk of Comparative Example2 and Example 3 as follows. A standard experimental setup was used tomeasure diffraction efficiency in a transmission geometry at awavelength of 532 nm. Similar characterization was also carried out at405 nm. For the sensitivity measurements, both the reference beam andthe signal beam were incident on the test sample at oblique angles of45° for hologram recording. The sample was positioned on a rotary stage,which was controlled by a computer. Both the reference and signal beamshad the same optical power and were polarized in the same direction(parallel to the sample surface). The beam diameters (1/e₂) were 4 mm. Acolor filter and a small pinhole were placed in front of the detector toreduce optical noise from background light. A fast mechanical shutter infront of the laser controlled the hologram recording time. In the 532 nmsetup, a red 632 nm beam was used to monitor the dynamics duringhologram recording. The recording power for each beam varied from 1 mWto 100 mW and the recording time varied from 10 ms to a few seconds. Thediffracted power from a recorded hologram was determined from a Braggdetuning curve by rotating the sample disc by 0.2-0.4 degrees. The powerused to read out the holograms was two to three orders of magnitudelower than the recording power in order to minimize hologram erasureduring readout.

Quantum efficiencies were measured for Example 3 and ComparativeExamples 1 and 2 as follows Samples of known thickness and concentrationwith absorption of about O.2 at the measurement wavelength wereprepared. The absorption spectrum of the sample was measured from 200 to900 nm. The sample thickness, laser power and the laser spot size at thesample surface were measured A bleaching experiment was then performedon the sample to obtain the instantaneous bleaching fluence F₀ at thewavelength of interest while the UVvis spectrum from 200 to 900 nil wasmeasured in short time intervals (0.1 s to 1 s). The bleaching processwas followed by monitoring the absorption at a suitable wavelengthdifferent from the exposure wavelength A standard experimental setup formeasuring quantum efficiency was employed The sample was exposed at thesame spot where the UVvis was measured with a uniform light beam ofknown intensity I and spot size. Typically the samples were illuminateduntil the teaching process was complete, as indicated by a steady state.The transmitted power P(t) during exposure follows an exponential decaycurve when the absorption of the sample is relatively low.

The absorption cross-section was calculated according to equation 1 andthe quantum efficiency according to equation 2.

The thermal stability of a holographic data storage medium may at timesbe a critical characteristic for long-term reliability. When thefabrication occurs via a molding process, the importance of thermalstability of the dye is even greater as molding temperatures may exceed200° C. In order for the dye to remain intact during all processingsteps of the media fabrication, its decomposition temperature needs tobe above the processing temperatures. The thermal stability of dyes istypically evaluated in a DSC experiment.

The quantum efficiency (QE) of the photo-rearrangement of a dye isdirectly related to its sensitivity. The higher the QE, the higher thesensitivity can be. A higher sensitivity of the data storage mediumallows for faster write times. Furthermore, if the photo-rearrangementof the dye has a low QE, much of the energy deposited in the material isconverted into heat, which can have detrimental effects on the gratingwriting process. The diffraction efficiency that can be obtained in aholographic data storage medium is a measure for its data storagecapacity. Same applications require high DE in order to function. In allcases, however, a high DE is advantageous.

The foregoing examples are merely illustrative, serving to illustrateonly some of the features of the invention. The appended claims areintended to claim the invention as broadly as it has been conceived andthe examples herein presented are illustrative of selected embodimentsfrom a manifold of all possible embodiments. Accordingly, it isApplicants' intention that the appended claims are not to be limited bythe choice of examples utilized to illustrate features of the presentinvention. As used in the claims, the word “comprises” and itsgrammatical variants logically also subtend and include phrases ofvarying and differing extent such as for example, but not limitedthereto, “consisting essentially of” and “consisting of.” Wherenecessary, ranges have been supplied, those ranges are inclusive of allsub-ranges there between. It is to be expected that variations in theseranges will suggest themselves to a practitioner having ordinary skillin the art and where not already dedicated to the public, thosevariations should where possible be construed to be covered by theappended claims. It is also anticipated that advances in science andtechnology will make equivalents and substitutions possible that are notnow contemplated by reason of the impression of language and thesevariations should also be construed where possible to be covered by theappended claims.

1. A method for storing holographic data comprising: (A) providing aholographic storage medium comprising an optically transparentsubstrate, said optically transparent substrate comprising aphotochemically active dye having at least two nitrone groups; and (B)irradiating the optically transparent substrate with a holographicinterference pattern, wherein the pattern has a first wavelength and anintensity both sufficient to convert, within a volume element of thesubstrate, at least some of the photochemically active dye into aphoto-product, and producing within the irradiated volume elementconcentration variations of the photo-product corresponding to theholographic interference pattern, thereby producing an opticallyreadable datum corresponding to the volume element.
 2. The method ofclaim 1, wherein the optically transparent substrate comprises athermoplastic polymer, a thermosetting polymer, or a combinationcomprising at least one of the foregoing polymers.
 3. The method ofclaim 1, wherein the optically transparent substrate comprises apolycarbonate.
 4. The method of claim 1, wherein the opticallytransparent substrate has a thickness of greater than about 100micrometers.
 5. The method of claim 1, wherein the photochemicallyactive dye comprising at least two nitrone groups has a structure (I)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical Q¹ is a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical, or a polymer chain and “a” is aninteger from 2 to
 100. 6. The method of claim 5, wherein thephotochemically active dye has a structure (II)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and “a” is an integer from 2 to
 4. 7. The method of claim 5,wherein the photochemically active dye is

wherein R³ and R⁴ are independently halogen, hydrogen, deuterium, aC₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀aromatic radical; and R⁵ is independently at each occurrence a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical.
 8. The method of claim 1, wherein the optically transparentsubstrate comprises the photochemically active dye in an amount fromabout
 0. 1 weight percent to about 10 weight percent.
 9. The method ofclaim 1, wherein the first wavelength is in a range from about 360nanometers to about 1500 nanometers.
 10. The method of claim 1, whereinthe first wavelength is in a range from about 400 nanometers to about650 nanometers.
 11. The method of claim 1, wherein the photo-productcomprises an oxaziridine, a rearrangement product of an oxaziridine, ora combination thereof.
 12. The method of claim 1, further comprisingstabilizing the holographic interference pattern.
 13. A method forstoring holographic data comprising: (A) providing a holographic storagemedium comprising an optically transparent substrate, said opticallytransparent substrate comprising a photochemically active dye havingstructure (I)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical Q¹ is a C₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphaticradical, or a C₂-C₃₀ aromatic radical, or a polymer chain and “a” is aninteger from 2 to 100; and (B) irradiating the optically transparentsubstrate with a holographic interference pattern, wherein the patternhas a first wavelength and an intensity both sufficient to convert,within a volume element of the substrate, at least some of thephotochemically active dye into a photo-product, and producing withinthe irradiated volume element concentration variations of thephoto-product corresponding to the holographic interference pattern,thereby producing an optically readable datum corresponding to thevolume element.
 14. The method of claim 13, wherein the opticallytransparent substrate comprises a thermoplastic polymer, a thermosettingpolymer, or a combination comprising at least one of the foregoingpolymers.
 15. The method of claim 13, wherein the optically transparentsubstrate has a thickness of greater than about 100 micrometers.
 16. Themethod of claim 13, wherein the optically transparent substratecomprises the photochemically active dye in an amount from about
 0. 1weight percent to about 10 weight percent.
 17. The method of claim 13,wherein the first wavelength is in a range from about 400 nanometers toabout 650 nanometers.
 18. The holographic storage medium of claim 13,further comprising stabilizing the holographic interference pattern. 19.A method for storing holographic data comprising: (A) providing aholographic storage medium comprising an optically transparentsubstrate, said optically transparent substrate comprising aphotochemically active dye having structure (II)

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and “a” is an integer from 2 to 4, wherein the dye is presentin an amount from about 0.1 weight percent to about 10 weight percent;and (B) irradiating the optically transparent substrate with aholographic interference pattern, wherein the pattern has a firstwavelength and an intensity both sufficient to convert, within a volumeelement of the substrate, at least some of the photochemically activedye into a photo-product, and producing within the irradiated volumeelement concentration variations of the photo-product corresponding tothe holographic interference pattern, thereby producing an opticallyreadable datum corresponding to the volume element, and wherein thefirst wavelength is about 500 nm.
 20. A holographic storage mediumcomprising at least one (i) at least one optically transparent plasticmaterial, and (ii) at least one photochemically active polynitrone dye.21. The holographic storage medium of claim 20 wherein said polynitronecomprises a thiophene-containing dinitrone compound having structure V

wherein R¹ is independently at each occurrence a C₁-C₂₀ aliphaticradical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromatic radical;R² is independently at each occurrence hydrogen, deuterium, a C₁-C₂₀aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀ aromaticradical; and R³ and R⁴ are independently halogen, hydrogen, deuterium, aC₁-C₂₀ aliphatic radical, a C₃-C₂₀ cycloaliphatic radical, or a C₂-C₃₀aromatic radical.